One of the methods for ensuring a reliable reception of a radio signal is by use of reception diversity. According to this method, shown in FIG. 1, the signal is received simultaneously by a multiplicity of antennas A, B, C, D. Each antenna is fed to a respective separate receiver 102A, B, C, D. Frequency sources 104A, B, C, D are used in the receivers for down conversion of the received signal. The down converted signals of all receivers are fed to a selection subsystem 106 which selects the best signal at any given moment. To ensure a proper operation of such system it is extremely important to ensure minimum deviation between frequency sources 104. Nevertheless it is also important to keep good absolute accuracy of all frequency sources. If these terms are not kept, the frequency errors might degrade the quality of the demodulated signal even though the communication link was adequate. In order to keep good accuracy and frequency correlation between the frequency sources of all receivers, in the state of the art an accurate frequency source is used in each receiver. Such accurate frequency sources are high quality, expensive oscillators with low aging rate and special means for stabilizing the output frequency versus temperature variation. With increasing required accuracy and stabilization requirements, the cost of the frequency sources increases as well.
The need for synchronizing several receivers on the same frequency is not new. Several methods are known in this field. The simplest method is based on the use of an external (to the system) frequency source, split the signal into branches and feed each receiver with a branch of the split signal. This method has several known drawbacks: the first drawback is due to the fact that the operation relies on a single frequency source. If this frequency source ceases to function, all receivers cease to function. In addition, there is no backup for the scenario where the relatively sensitive phase lock loop in each receiver ceases to function. If any phase locked loop ceases to function, there is no identification for this situation and the corresponding receiver also ceases to function. In addition, the solution described above does not deal with the common problem of disconnected cables. If any of the cables connecting the frequency reference to the receiver is disconnected, the corresponding receiver also ceases to function.
U.S. Patent Application 2005/0104666 discloses a typical example of a prior art mechanism for synchronizing frequency sources of multiple receivers. According to US 2005/0104666, a single frequency source synchronizes two or more phase locked loops, each generating a frequency used by a different receiver. One phase locked loop (“master”) synchronizes the other phase locked loops at any given moment. The system described in US 2005/0104666 has the ability to overcome sharp deviations of the master phase locked loop from the nominal frequency. When such a deviation is identified, another phase locked loop takes control and synchronizes the other phase locked loops. However, this system has several drawbacks that the present invention solves. The operation of the system described in US 2005/0104666 is based on a single reference frequency. If this reference frequency ceases to function, all phase locked loops loose their frequency reference and deviate from their central frequency with no means to rectify this situation. In other words, this system has no redundancy in case the reference frequency source ceases to function. In addition, if any of the cables connecting any of the phase locked loops to the frequency reference or connecting the slaves phase locked loops to the master phase locked loops are disconnected the corresponded phase locked loop cease to operate.
Accordingly, there is a need for, and it would be advantageous to have a mechanism to ensure proper synchronization of frequency sources without use of very expensive frequency sources and to create a spontaneous detection and recovery mechanism from several potential failures.